NEW YORK — John and Sue have an offbeat marital arrangement. For the last five years of their marriage, Sue has spent three nights a week with her boyfriend, Fred.

And that’s not even the strange part.

As it turns out, John openly shares Sue — and their king-size marital bed — with Fred. Confused? Consider this: During the rest of the week, Fred sleeps at home with his wife, Peggy, and their male lover, Bill.

John, a 71-year-old San Francisco-based researcher, also has relationships outside his marriage to Sue. He has three current girlfriends, Fred has two and John’s wife has four boyfriends. He even refers to Sylvia, the sister of one of his wife’s lovers, as “my sister in love.” Following along?

While John has nothing against monogamy, he said, “You have to spend a lot of energy to be monogamous.”

It’s hard to imagine a more energetic bunch than John, Sue and their various lovers, who belong to a growing movement of Americans who practice polyamory — intimate long-term relationships with more than two people. (The polyamorous lovers in this story requested anonymity because of fears of discrimination from employers and even friends.)

Believers in polyamory estimate their numbers in the tens of thousands; the attorney general’s office in Utah and Arizona have calculated that up to 40,000 Americans practice polygamy, a form of polyamory limited to multiple spouses.

The subject of polyamorous relationships will likely become the stuff of water cooler conversation on March 12, when HBO debuts “Big Love,” its new Sunday-night drama, in which the central character, played by Bill Paxton, lives a normal suburban life with his three wives in three separate homes. “It’s really an odd way to look at marriage,” says Mark Olson, co-creator of the television series.

“Poly” has become the umbrella label to describe those who maintain relationships with multiple partners. The word “polyamory” is so new that neither the Oxford English Dictionary nor Webster’s includes it. In the last decade, there has been a mini-explosion of “poly” terminology to describe multipartner relationships like polyfidelity (no sex outside of a group), polymono mix (one partner is polyamorous, the other monogamous) and polyfamily (which refers to shared child-rearing).

Monogamists often confuse polyamory with swinging, a practice that gained notoriety in the 1970s. Robyn Trask, managing editor of Loving More, a magazine based in Boulder, Colo., devoted to the poly lifestyle, defines polygamy as simply “monogamy with more people.” Other defenders point to the fact that practicing “polys” don’t engage in one-night stands.

“Poly is about establishing relationships,” said Sasha Lessin, co-founder of the Polyamory World Association, based in Maui, Hawaii.

Forming these complicated and controversial relationships can be difficult. The Internet has opened up many people to the poly lifestyle over the last decade, strengthening their numbers and connections.

“The Internet has brought the polyamorous community together like nothing else,” Trask said. She added that polys can now anonymously seek out others through commercial personal ad sites ranging from PolyMatchmaker to free message boards from Arizona to Kansas to Virginia.

But for every poly that chats online, joins a listserv or posts personal ads, others remain hidden and closeted.

“Thirty years ago, it was considered shameful if couples lived together outside of marriage, and to have a baby unwed was scandalous,” said Valerie White, executive director of the Sexual Freedom Legal Defense and Education Fund. “Now celebrities have children out of wedlock.”

While no laws exist to prohibit polyamory, many polys struggle with legal issues such as property co-ownership and child custody. In 1999, April Divilbiss, then a 22-year-old Tennessee resident, lost custody of her daughter after she outed herself as a polyparent on an MTV documentary.

Leaders in the movement insist that polys live all around us and work in mainstream jobs. One female poly practitioner cites a judge, a rabbi and a Nobel Prize-winning scientist as polys. How does the woman know about the scientist? “He’s one of my lovers,” she said.

Polys haven’t yet figured out how to escape the stigma attached to their lifestyle.

“It’s easier to come out of the closet as gay, bi, kinky, even Republican than poly because you’re challenging the foundation of everybody’s relationships,” said George Marvil, co-organizer of PolyLiving 2006, a poly conference held in Philadelphia in early February.

Critics call it by another name

And that’s exactly why others find polyamorous relationships objectionable. Critics know it by a different name: cheating. In 2002, the National Opinion Research Center at the University of Chicago found that 15 percent of wives and 22 percent of husbands admitted to having sex with someone other than their spouses.

Those statistics feed the poly defense. “Monogamy is ideal as a standard, but if you look around you, it’s not the end result,” said Justen Michael, 32, founder of Polyamorous NYC, a social group in New York.

It turns out that even polyamory doesn’t relieve the impulse to stray. When Trask confronted her husband about sneaking around with a long-distance girlfriend for three months, he denied it. After she confronted him with cell phone bills, her husband confessed. “There was no reason not to tell me,” Trask said. The couple is now separated and plans to divorce.

With so many partners, time management can also become a problem. “Love may be unlimited, time is not,” Trask said. “One of the poly skills we teach is calendaring.”

It may sound like fun, but even its biggest defenders admit that the poly lifestyle isn’t for everyone.

“There’s a lot of roadkill on the road to polyamory,” said Ken Haslam, a 71-year-old full-time poly activist from Galena, Md.

Still, some younger participants disagree. Birgitte Philippedes, 38, a poly activist from New York, says “Poly’s about having choice that can be a beautiful self-expression.”

Her friend Sydney agrees. “It’s not about having sex with my friends,” she said. “It’s about having the option to have sex with my friends.”